Various methods and apparatus have been known for grinding particulate solids in a liquid media. They include ball mills, pebble mills, roll mills, sand mills and agitated-media mills. Illustrative art is believed to be U.S. Pat. Nos. 1,577,052, 2,764,359, 3,903,191, 3,008,657, 3,131,875, 3,298,618, 3,149,789, 3,204,880, 3,337,140, 3,432,109, 3,591,349, British Pat. Nos. 716,316 and 1,038,153, and German Pat. Nos. 1,214,516 and 1,233,237.
An agitated-media comminuting apparatus commutes the solids in liquid suspension by subjecting them to generally random contact in a bed of agitated grinding elements. In an agitated-media comminuting apparatus, the grinding is usually performed in a vertical cylindrical stationary tank or vessel with a rotatable agitator disposed on a substantially vertical axis. The agitator has one or more solid protuberances such as arms or discs extending out from the axis thereof into a mass of grinding media or elements such as pebbles, ceramic balls or metal balls that occupies a substantial portion of the vessel. The rotation of the protuberances through the mass of grinding media causes the media to occupy an increased apparent volume with the result that the grinding elements have a substantial free space between them and impinge on each other in a manner somewhat similar to the classic model of a gas. The particulate material to be ground and the liquid, which serves as a carrier and dispersing media for the material, occupy the spaces between the grinding media. The material is ground by the action of the agitated grinding media. A built-in pumping action may be used to maintain circulation within the comminuting means during comminution.
It has been recognized that more mass of material may be comminuted when the size of the comminuting vessel is increased. Agitated-media apparatus of larger size are, however, relatively more expensive to build and operate. It has also been recognized that the large capital outlay for larger size equipment could be avoided by recycling the suspended solids in the liquid continuum between a comminuting means and a large storage tank, see, e.g., U.S. Pat. No. 3,204,880. Recycling has been, however, generally regarded to result in increased grinding time to reduce the particles to a predetermined size. Such recycling operation results in large particles appearing in the final product without having been subjected to a sufficient amount of grinding.
The present invention claimed in the parent application overcomes these difficulties and disadvantages of the prior art above described. It has been found, contrary to what would be expected, that recycling at relatively high flow rates results in reduced grinding times. It would be expected that the grinding time in a recycling operation would never be as short as the grinding time for the same batch of material ground in a grinding means equal in volume to the retaining tank plus the comminuting means without recirculation, or to a series of grinders equal in total volume to the volume of the retaining tank and comminuting means without recirculation. However, applicant has found that, surprisingly, the grinding time for a given batch reduces with relatively high recirculation rates to less than the time to grind the same batch in a single grinding means, or a series of grinders equal in volume to the retaining tank plus comminuting means. The processing capacity of a given size grinding apparatus is thereby increased without the necessity for relatively large and expensive grinding apparatus. The present invention claimed in this application is directed specifically to the processing of chocolate where the invention has special application and advantages.
Chocolate Making: Chocolate is made by comminuting and mixing various compositions of chocolate solids in a liquid continuum. After winnowing, the nib is ground to produce cocoa mass or chocolate liquor (sometimes called "bitter chocolate"), from which cocoa butter can be extracted by pressing. On cocoa butter production, the press cake is pulverized to produce cocoa powder. The extracted cocoa butter is used in the preparation of chocolate by incorporating it into a mixture of sugar and nib and/or chocolate liquor to produce sweet chocolate (sometimes called "plain chocolate"), and into a mixture of sugar, milk powder and nib and/or chocolate liquor to produce milk chocolate (called "milk chocolate").
Chocolate Liquor: The nib is a cellular mass containing about 55 percent cocoa butter (i.e., cocoa fat) locked within the cell structure. When the cell walls are ruptured during comminuting, the fat is released and, as it is liquified by the frictional heat, it becomes a continuous phase in which the disintegrated cell particles (i.e., chocolate solids) are suspended. As disintegration proceeds, more and more fat is liberated, reducing the viscosity of the paste until it becomes a fluid which is referred to as chocolate liquor (or cocoa mass).
A variety of grinding devices have to be used to perform the grinding and mixing in the various stages of chocolate making. For chocolate liquor production, originally circular stone mills consisting of typically three sets of horizontal double stone plates were widely used; in each set, one stone was held stationary while the other is rotated in close contact with it, and the materials to be ground are fed through the center opening and passed to and collected at the outer periphery. The grinding action was improved by cutting grooves in the stones to distribute and regulate the feed. These machines have been replaced in many plants by steel disc mills, with the discs mounted either horizontally or vertically, and with a central steel disc rotating at high speed between two stationary steel faces, all of which are suitably grooved; the material to be ground is again fed to the center of the discs and passed to and collected at the outer periphery. Chocolate liquor is also commercially processed by cylindrical four- and five-steel roll refiners. In any case, the control of the size and uniformity of the chocolate solids in the liquor (as well as processing temperature, discussed below) is critical to further processing and taste and appearance of the final chocolate liquor product.
The comminuting of chocolate liquor generates considerable heat and temperature can rise to 105.degree.-110.degree. C. The frictional heat generated during comminution will defeat the purpose of light roasting for sweet chocolate production; therefore, water cooling is generally provided on the steel disc mills and comminuting devices. In this manner, by controlling the comminuting rate and the cooling, the chocolate liquor temperature can be controlled during comminution and generally maintained between 100.degree. and 220.degree. F. During the grinding stage, no chemical changes are desired or generally involved, although there may be some loss of moisture and acetic acid vapor from the mass.
Cocoa Butter and Cocoa Powder: The fineness and uniformity of the chocolate solids in the liquor is also very important in all chocolate products, although more important in making some chocolate products then in making others. Where the liquor is used directly in making sweet or milk chocolate, fineness and uniformity of particle size is not as important because of the subsequent processing involved during refinement with sugar particles, or sugar and milk particles. For cocoa powder or cocoa butter production, it is, however, desirable to have the chocolate solids of the liquor as fine and uniform as possible. This provides the maximum, controlled amount of the cocoa butter in free state, for efficient extraction of the butter. Fineness and uniformity of the particles is also necessary to facilitate dispersion and provide taste and color to the cocoa powder where the powder is used for production of products such as beverages, pudding mixes, ice cream, syrups and sauces. The particle size of the chocolate solids in the liquor during production are generally measured by micrometer measurement, by test sieving, and by sediment methods.
Cocoa powder is produced from the chocolate liquor by removing from it some of the cocoa butter, generally by hydraulic pressing, and grinding and pulverizing the resulting press cake. The preparation of cocoa powder from the press cake involves several factors not usually associated with grinding and pulverizing operations. The remaining cocoa butter presents the greatest difficulty during this operation. A temperature rise, resulting from the heat generated during grinding and pulverizing, to above 30.degree. C. will soften the cocoa butter, and above 34.degree. C. will melt it. The cocoa butter can then spread in a thin layer on the surface of the cocoa powder particles, causing them to stick together, in turn cushioning them against the shattering effect of the grinding machine. It is necessary to use cold, dry air in sufficient quantities to remove the heat generated by grinding and to cool the powder so that the butter will form a solid stable phase within the cocoa particles. If this is not achieved, instead of a fine cocoa powder, rich in color and maintaining its free flowing properties when packed, a powder will be produced which will be a dull color and be more liable to cake. Further, most cocoa powders are subjected to alkali treatment at some stage in their manufacture to improve their color, flavor and dispersibility. Fine, uniform particle size of the chocolate particles is important to efficient, effective alkalization.
Most types of hammer mills, impact mills and agitated-media mills have been used for grinding and pulverization of cake to form cocoa powder. Usually, these mills have some type of built-in grading system, such as mesh screens, cones, or sieve classifiers to ensure the desired degree and uniformity of fineness. Recycling arrangements also have been used to remove and reprocess coarse particles. Sieves and classifiers with aperture sizes of between 100-250 microns are commonly used in grinding the cocoa powder. The fineness and uniformity of particle size in the cocoa powder is an important property in all cocoa since, apart from its effect on taste, fineness and uniformity controls the rate of dispersion, homogeniety of color and rate of sedimentation of the chocolate particles in liquor products.
Refined Chocolate: One universal requirement of refined chocolate, whether bitter, sweet or milk chocolate, is that it should give no sensation of roughness when eaten. It has been shown that the palate cannot discriminate individual particles below about 30 microns in size, so it is essential that combination of the sugar and chocolate solids be carried to the point where only a small portion of the particles is above this size, see H. M. Mason, Analyst 58, 444, (1933). Conversely, it is generally critical to chocolate processing, particularly molding, extruding and enrobing, that the viscosity of the chocolate be maintained relatively high, requiring controlled and maintained particle size uniformity above about 10 microns. These requirements place a premium on the control and maintenance of a small, substantially uniform solid particle size in the chocolate.
At one time, the complete size reduction was achieved by the prolonged mixing of cocoa nibs with granulated sugar and cocoa butter in "melangeurs" of mortar-mill type with heavy granite rollers running on a revolving granite bed. Several days were required for the production of sufficiently fine chocolate. More recently, after a brief initial mix in the melangeur or other suitable mixer, the coarse paste is passed through a series of roll refiners. These comminuting machines may have from three to six water-cooled, hollow steel rolls, one above the other, and passage over as many as four sets of such refiners may be required to give the necessary fineness. As the surface area of the solids increases during grinding, an increasing amount of fat is required to maintain the fluidity of the paste being ground. Provision is therefore made for mixing in more cocoa butter, where necessary, between the comminuting stages. Modern refiners have capacities of up to 1800 pounds per hour and may be fitted with hydraulic devices for maintaining controlled pressure between the rolls, the pressure being indicated on gauges.
It is now common practice to use, in the preliminary mix, ingredients previously finely ground so that the number of refining grinds can be reduced. Starting with a mixture of chocolate liquor and sugar (pulverized in machines of the hammer-mill type), it is possible to obtain a sufficiently smooth final product by only a single-passage over the roll refiners.
Conching: Apart from mixing with more cocoa butter to provide the correct consistency for molding or enrobing, and addition of the desired flavoring material, some of the cheaper chocolates receive no further processing. All better-quality chocolates, however, undergo the treatment known as "conching" (so-called from the shell-like form of the original machines) to aerate the chocolate and remove undesirable components, such as acetic acid. The earliest type, and from a flavor standpoint still probably the most successful, is the longitudinal conche which consists essentially of a tank with a granite bed on which chocolate is pushed to and fro by a roller traveling backward and forward. The end of the conche is so shaped that chocolate that forced against it is splashed back over the roller into the body of the machine. The temperature of the chocolate during conching is 55.degree.-85.degree. C. for plain chocolate, but is lower for milk chocolate, generally in the range of 45.degree.-55.degree. C.; higher temperatures are sometimes used for the development of caramel-like flavors. The time taken may be twelve hours or less, particularly for milk chocolate. The viscosity of the chocolate is reduced during the early stages of conching.
The present invention overcomes the difficulties and disadvantages of prior art devices and methods. It provides a universal method for making chocolate liquor, chocolate butter and cocoa powder, and refined chocolate of superior quality and properties more efficiently and inexpensively. It reduces and often totally eliminates the need for water cooling during processing and can provide conching in the course of comminuting. It also reduces the amount of cocoa butter needed to make semi-sweet and milk chocolate. And most importantly, it provides better, sweet and milk chocolate and cocoa powder of controlled uniformity and fineness than heretofore attained in chocolate making.